The goal of this research project is to validate in humans a new ligand with picomolar affinity for the serotonin transporter. It will permit quantitative measurement of serotonin transporter (5- HTT) sites in living brain by imaging with single photon emission tomography (SPECT). The serotonin transporter is known to play a critical role in several physiological and pathophysiological processes, including depression, anxiety, seasonal affective disorder, bulimia, and psychosis, as demonstrated by post-mortem data, clinical efficacy of therapies affecting this target site, and imaging studies. A radiotracer that binds specifically to 5-HTT is of great utility in diagnosis, monitoring treatment, and research of these disorders. There is a need for better tracers that allow quantitation in areas of the brain with low 5-HTT densities such as the cortex.